Forward, push-push, flyback and similar converter topologies require that the power transformer core be periodically reset in each cycle of operation. The core reset voltage is supplied by a voltage reset capacitor which is periodically charged each cycle to provide the needed reset voltage.
The voltage due to charge stored on the reset capacitor is dependent on the duty cycle of the converter's power switch. Accordingly, the circuitry is designed so that the proper reset voltage is stored by the reset capacitor during normal operation (i.e., normal duty cycles)
The input voltage to the converter may drop below the low limit operating voltage for which the converter is designed. These low input voltage conditions can be caused by "brownout" conditions in the ac mains, or batteries discharged beyond their capacity, or system faults. At low input voltages, outside the normal designed input voltage range of the converter, the duty cycle of the power switches tends toward unity. This causes abnormally high voltages to be supplied by the reset capacitor, which may cause a failure of the converter circuit.
Similarly, if the input voltage rises above the high limit voltage of the input voltage range, the reset capacitor can attain a very high voltage level which may also destroy components of the converter.